1. Field of the Invention
This invention relates to hot melt sticks based upon moisture-curable hot melt compositions.
2. Description of Related Art
Moisture-curable hot melt compositions are adhesive, coating, or sealant compositions that are solid at room temperature, but melt at elevated temperatures and then cure in the presence of moisture to produce a crosslinked thermoset resin. Examples of such compositions include isocyanate-terminated polyurethane prepolymers and silyl-terminated prepolymers. The isocyanate or silyl terminal groups react with moisture to produce a cured, thermoset resin. Moisture-curable hot melt compositions are desirable because although they are applied like conventional thermoplastic hot melt compositions, the final cured product exhibits superior properties (e.g., tensile, shear, and peel strength), especially at elevated temperatures.
Due to their moisture sensitivity, it is customary to use complex storage and handling procedures with moisture-curable hot melt compositions. Typically, the composition is stored prior to use in a sealed metal canister under an inert atmosphere. Applicators have been developed which utilize heated platens that are placed into the top opening of the canister to melt the top layer of the moisture-curable hot melt composition for delivery using a pump. These applicators are expensive and utilize a complex sealing arrangement in order to exclude moisture during application. Moreover, it takes time to remove the platen from the canister when it is time to change canisters.
Other applicators have been developed where the entire contents of the canister are heated in order to convert the composition into a suitable form for application. This may be accomplished by transferring the canister to a pre-heater. Following pre-heating, the canister is transferred to the hot melt applicator. Alternatively, the canister may be placed directly in the hot melt applicator and heated there; this process, however, is time-consuming. In contrast, non-moisture sensitive thermoplastic hot melt compositions can be used in "melt on demand" applicators; that is, they are prepared in the form of a stick which is simply supplied to the melting chamber of the applicator without pre-heating or moisture-exclusion for direct application.
Attempts have been made to improve the storage and handling of moisture-curable hot melt compositions. For example, Japanese Patent Application No. 3-36015 describes providing a moisture-curable hot melt composition with a 5-200 micron thick moisture-resistant outer coating to improve shelf life. The hot melt composition and outer coating are co-extruded to form a composite structure in the form of a bead, strand, or pellet. Examples of suitable materials given for the outer coating are polyethylene, polypropylene, polyvinyl chloride, polyvinylidene chloride, paraffin wax, and natural waxes. While the Japanese Patent Application states that in use the core hot melt composition and outer coating can be melted together and applied, it further states that it is preferable to strip the outer coating from the core prior to application. Although the Japanese Patent Application mentions ethylene-containing copolymers such as ethylene/vinyl acetate, ethylene/ethyl acrylate, and ethylene/methyl acrylate copolymers, alone or in combination with tackifying resins, they are suggested for combination with the moisture-curable hot melt composition in the core of the composite, rather than for use as the outer moisture-resistant coating.
European Patent Application No. 0 469 564 A1 describes providing both thermoplastic and thermosetting hot melt compositions with a non-stick thermoplastic envelope that is compatible with the hot melt composition to improve storage and handling; moisture-curable isocyanate-terminated prepolymers are given as an example of a thermosetting composition. A wide variety of thermoplastic resins are suggested for the envelope, including ethylene/vinyl acetate and ethylene/methyl acrylate copolymers. The composites are provided in the form of relatively large pillows or sacks suitable for use in a hopper-fed dispenser.